I am not qualified to teach anyone about highway engineering, I have a trade-based background, and am only providing references to what you may find helpful in understand counting traffic and the limits of a car lane.

The first information comes from a book you can find in Wellington City Library, called ‘Wellington, a sick city’ by W.B. Sutch, published in July 1966.

Researchers for General Motors (hardly biased against motorways) found that “single lane traffic in various vehicular tunnels show this best speed to be about 20 miles per hour. At higher or lower speeds the volume drops”. In this test it was found that speeds as slow as 15 miles an hour enabled more traffic to pass than speeds of 60mph. The actual count: at 15 miles per hour, 1,176 vehicles passed a given point; at 35mph, the figure was 1,464; and at 60mph only 1,152. In other words a speed of 30 mph accommodated almost 30 percent more traffic than a speed of 60mph. Motorways for a city are as out of context as a dinosaur.

To find “rules of thumb” you will need to click ‘1.7 Integration and Segregation’ then ‘1.7.4 Guidance Graph’ then ‘3’. Traffic Volumes – some rules of thumb”
. What this helps us to understand is the difference between peak traffic flow and AADT (Annual Average Daily Traffic).

– Traffic flow should be stable with about 440 cars per hour at a speed of about 30 mph (48 km/h)
– At 760 vehicles per hour flow starts to become unstable with speed dropping to 24 mph [38 km/h]
– At 830 vehicles per hour flow becomes borderline unstable with a speed of about 19mph (30 km/h)

And this shows the hour of the day we design our road for

Note: road building is expensive.
The question to ask when we see extra lanes on a road is “what justifies this extra lane, and the extra cost?”

Related

5 comments on “Number of cars per lane”

[…] Going back to the Frequently Asked Questions – Why are you only building two lanes?The Cambridge bypass predicted traffic flow for 2021 is about 12,600 vehicles per day or 6,300 vehicles per day per each direction. I am suggesting a possible 840 vehicles per hour per lane at peak periods using the rule of thumb from this earlier post – number-of-cars-per-lane […]

Peter H

August 30, 2014 at 12:03 am

We have a piece of infrastructure capable of moving 1200 vph to 1400 vph per lane, but ideally not more than 760 vph. What the above graph shows is a road being under used for 95% of the day and over used for 5% of the day.

This is not a new issue http://www.eyewitnesstohistory.com/ford.htm
“It was the great common sense that Mr. Ford could apply to new ideas and his ability to simplify seemingly complicated problems that made him the pioneer”
“What was worked out at Ford was the practice of moving the work from one worker to another until it became a complete unit, then arranging the flow of these units at the right time and the right place”

If we are under using a road 95% of the day, we do not have a shortage of infrastructure; we have an issue with timing of flow and types of transport units.